The design and construction of passenger transit vehicle doors is crucial for the safety of the travelling public. These doors must function in a hostile environment which at least includes heavy usage, relatively large temperature extremes, vibration loads, acceleration loads and spurious electrical signals. Such spurious electrical signals may be caused by lightning, by interrupted contact with a third rail, and/or strong radio signals. They must function reliably for a very large number of cycles. For many applications, they must be designed to function rapidly and safely without the direct observation thereof by an operator of the transit vehicle.
The doors should have locks so that they cannot be opened by application of opening forces to the doors when the doors are closed and locked without energization of the locks in the unlocking direction. For safe and reliable operation, the doors also require sensors which indicate whether or not they are locked in the closed position. No credible failure mode should cause a door to open sufficiently for a person to fall out of the transit vehicle when the transit vehicle is moving.
It is generally necessary for the doors to have a manual unlocking means so that in the event of a failure of a control system which controls a door, or failure of an unlocking actuator, the door can be manually unlocked for emergency egress from the transit vehicle. It is desirable for the unlocking means to first unlock the door and to then move the door a small distance in the opening direction to provide a grip for a person to open the door fully.
Some prior art door locks use the mechanism which opens and closes the door to keep the door closed. This approach has the disadvantage that when the door is closed and locked forces on the door in the door opening direction are borne by the opening and closing mechanism. Some of these prior art door locks have the further disadvantage that if a spurious signal is received to open the door the door will unlock and open.